Tropical Moisture to Inundate the Northeast This Weekend into Next Week

Soak in the sun while blue skies and low humidity dominate the weather through Friday. An about-face this weekend will crank up the humidity, the clouds, and rain chances through much of next week in the Northeast. While all-day washouts are not expected, afternoon showers and thunderstorm chances could spoil outdoor plans until the pattern changes.

The culprits behind this period of unsettled weather will first be a coastal low at the surface followed by the continued presence of a cutoff low aloft. A cutoff low is a low pressure system stretching from the surface to the upper levels of the troposphere that is “cut off” from the flow of the jet stream. They can stay idle for days, bringing unsettled weather downstream from their center (to the east in the middle latitudes) until they dissipate.

A complicated pattern will ensure that the unsettled conditions persist for several days. The dominance of two upper-level ridges, one in the Four-Corners region and one over the western Atlantic Ocean, will draw tropical moisture northward and block an upper-level shortwave from reaching the coast. On Thursday the shortwave will cross into the Upper Plains from Canada and ride the northwesterly circulation around the ridge of high pressure centered over the Four-Corners region. The contract between the cool Canadian air and hot desert air masses will intensify the upper-level low, which will indubitably result in violent thunderstorms across much of the Midwest.

Early Saturday, the upper-level low will halt over the Ohio River Valley by a wall of high pressure imposed by the intense Bermuda High over the western Atlantic Ocean. Meanwhile, low pressure centered over the northern Gulf Coast will be pulled northeastward by the Bermuda High toward the Carolina coast ahead of the trough in the Ohio River Valley. This coastal low will ride up the coast and bring with it a river of moisture, forced into the Northeast by the complimentary flows of the Bermuda High to the east and the trough of low pressure to the west. The coastal low will barrel northward Sunday just before the trough to the west “cuts off” from the jet stream and becomes a cut-off low.

The coastal low will provide the initial round of rain Saturday afternoon into Sunday from North Carolina to Maine. Without the support of a jet stream, this coastal low will intensify very little. Intensification will not be necessary to deluge the Northeast, however. Robust warm and moisture advection from the Tropical Atlantic will overspread the region. This should be sufficient to produce widespread downpours in the vicinity of the coastal low.

There are some signs that the coastal low could develop tropical characteristics, notably a warm-core center. This is possible as it traverses the Gulf Stream, but development into a tropical depression is highly unlikely. The waters off the Northeast coast are not warm enough to sustain a tropical cyclone. Nonetheless, its brief time over the very warm Gulf Stream will further the development of heavy downpours around the low’s center.

The heaviest rain should develop close to the coast in the Mid-Atlantic Saturday afternoon into the overnight hours, but some heavier showers could reach northern New England and the North Country of New York Sunday as the coastal low tracks northeastward. The coastal low has the potential to locally drop over 2″ of rain over a few unlucky areas, but most areas are unlikely to receive more than 1.5″ of rain. Fortunately, the region will be spared from widespread flooding by the tightening pressure gradient between the Bermuda High to the east and the upper-level low to the west. This will cause the low to barrel towards Canada at a swift pace. By midday Sunday the showers from the coastal low should have cleared everywhere except northern New England.

There will not be much time to dry out in the Northeast before the next round of rain. High humidity and warm temperatures in the mid and upper 80s along the coastal plain Sunday afternoon will keep the environment ripe for the development of more showers and thunderstorms if provided a trigger. If the sun were to peek through the clouds Sunday behind the coastal low, the sun’s radiation at the surface could be enough to trigger convection and spawn more showers and thunderstorms.

Rain chances will continue through the work week as the atmosphere will remain water-logged. With nowhere to go, the upper-level low will cut off from the jet stream and weaken as the air masses underneath it, behind it and ahead of it combine. The ultimate result of this process will be the stretching of the cut-off low southward, which will serve to construct an atmospheric river directly from the Caribbean to the Northeast US. Lift from the cutoff low along with any amount of sunshine will be enough to spawn scattered thunderstorms each afternoon, possibly through next Friday.

Tuesday night the cutoff low will finally be confined to the Gulf of Mexico as it withers into a gentle circulation. Unfortunately, the force replacing the cutoff low will be another instigator of rain. Another upper-level trough will push through the East Wednesday through Friday. This trough will be much broader than the cutoff low. Continuing tropical flow from the Caribbean will spark showers and thunderstorms along its southern and eastern flanks, mainly near the Great Lakes Wednesday, but more widespread across the Northeast Thursday. A late week cold frontal passage could finally end the stormy weather by Friday.

Any storms that develop this weekend and next week will be capable of producing torrential downpours that could drop large amounts of rain in short periods of time. It may not rain at the same location every day, but when it does rain it will likely pour in large quantities. Over the course of the week, guidance is in substantial agreement that rain will accumulate at least two inches throughout most of the Northeast. The 00 UTC July 18 ECMWF ensemble (EPS) predicts with up to 99% certainty that the I-95 corridor will pick up at least 2″ of rain through next Friday. Some ensembles like the American GEFS, not depicted here, show a more widespread distribution. That almost all of the 51 members of the EPS depict so much rain through next week is statistically remarkable, and permits unusually high confidence about heavy rain so far in advance.

ECMWF EPS probability of exceeding 2″ of rainfall between 8pm July 17 and 8pm July 27. Graphic courtesy of weather.us.

The one fortunate result of the extended period of unsettled weather is that extended periods of heat are not likely. Temperatures will be around normal during this period. The I-95 corridor could see temperatures around 90 a few days next week as it resides along the outer edge of the Bermuda High, but the heat will be moderated by the thunderstorms.

With three days until the start of the wet pattern, and more than a week until its end, there is still uncertainty as to when, where, and how much it will rain. Be sure to keep checking back for updates as we know more.

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.